This invention relates to systems for controlling the energizing of electrical resistance igniters for proven ignition systems.
In many gas-fired devices, standing pilots have been replaced by either hot surface igniters or spark ignition devices. While spark ignition devices provide rapid ignition, they generate undesirable electrical and acoustical noise. In applications that use a microprocessor, such electrical noise is undesirable since it can adversely affect operation of the microprocessor. In addition, spark devices may be difficult to predict as to generation of an arc proven to ignite gas. Hot surface ignition does not generate such electrical or acoustical noise, but does require careful control of its temperature to prevent damage to the igniter.
An igniter known in the art that is capable of warming up quickly is a silicon nitride igniter, an igniter constructed of a tungsten alloy heater element embedded in a silicon nitride insulating material. While such an igniter is desirable because of its mechanical strength and durability, it has a critical temperature limitation, which must be adhered to. Specifically, the silicon nitride igniter must remain below approximately 1300xc2x0 C. If the igniter temperature repeatedly approaches 1300xc2x0 C., the igniter will prematurely fail, such failure generally consisting of the opening of the tungsten heater element. Since a temperature over 1100xc2x0 C. is required to reliably ignite gas, the igniter must operate within a narrow temperature span between the lowest temperature that will reliably ignite gas and the highest temperature that the igniter can withstand.
U.S. Pat. No. 4,925,386, assigned to the assignee of the present invention, discloses a learning routine for energizing an igniter to a temperature above the minimum ignition temperature, and successively energizing the igniter to a slightly lower temperature during each successive cycle until it fails to ignite the gas. After an unsuccessful ignition attempt, energy to the igniter is increased so that the igniter operates just above the lowest possible ignition temperature, which prolongs igniter life. While an occasional unsuccessful ignition attempt is generally acceptable in many applications, it is not acceptable in commercial applications of high BTU output that require a proven method for igniting gas.
U.S. Pat. No. 5,725,368, assigned to the assignee of the present invention, discloses a system for determining the level of power to be applied to the igniter based on the value of the voltage available to energize the igniter and on the value of the resistance of the igniter. A triac in series with the igniter is fired in an irregular firing sequence, which is determined from a look-up table in a microcomputer. Specifically, the look-up table enables selecting a firing sequence based on the determined value of line voltage and the determined xe2x80x9ccoldxe2x80x9d resistance value of the igniter. But due to manufacturing tolerances, the xe2x80x9ccoldxe2x80x9d resistance of the igniters can vary considerably from igniter to igniter. In addition, the xe2x80x9ccoldxe2x80x9d resistance of each igniter can vary considerably from cycle to cycle as a result of residual heat in the igniter. Such variances are difficult to accurately compensate for. While prior art systems are useful for the purposes described, there is still a need for a method of accurately controlling the power and temperature of an igniter that is used where a proven source of igniting gas is required before the gas supply is opened.
The present invention relates to a microprocessor-controlled triac-switching circuit for a silicon nitride igniter, having line voltage and igniter current input means to the microprocessor for determining power to the igniter. Upon determining the line voltage, the microprocessor uses a look-up table to select a corresponding triac-switching sequence to drive a igniter to a power level proven to ignite gas via empirical testing on the application. The igniter current resulting from the switching sequence is fed to the microprocessor, which determines the actual power level and an offset value. The offset functions as a compensation means, shifting the sequences in the look-up table to achieve the desired power to the igniter. After a predetermined warm-up time, the microprocessor sends a signal indicating whether the igniter is at a power level proven to ignite gas.